Add Per-Thread stack guard overflow check

[xuchang-vivo]

This PR adds per-thread MPU stack-guard switching support for Cortex-M and completes the related validation/documentation.

    /// ------------------------------ <--- stack.top() == allocated buffer end
    /// | optional align gap (0..A)  |
    /// ------------------------------
    /// | Context for current thread |
    /// ------------------------------ <--- saved_sp (initial SP)
    /// |       stack region         |     (stack grows down)
    /// |           ...              |
    /// ------------------------------ <--- guard upper bound (if enabled)
    /// | stack guard region         |     (optional, usually no-access)
    /// ------------------------------ <--- stack.base()

What changed

• Added per-thread guard programming in ARM v8-M MPU via update_thread_stack_guard(next).
• Wired guard updates into the context-switch path, right before restoring the next thread context.
• Wired guard setup into the initial scheduling path, so the first running thread gets a correct guard before entering thread mode.
• Fixed guard alignment/range handling:
  • Guard start is aligned up to 32 bytes.
  • Guard region is constrained to stay within the target thread stack range.
  • This avoids protecting memory below stack_base.
• Added a new unit test in kernel/src/lib.rs to verify that writing into a thread stack guard triggers MemManage.
• Updated stack-layout comments in thread::init to clarify guard placement and fault behavior when SP is near/in guard on exception entry.

Verification

Executed and passed:

• ninja -C out/qemu_mps3_an547.debug kernel/kernel:run_unittest
• ninja -C out/qemu_mps3_an547.debug check_all

Relevant tests passing include:

• test_mpu_sys_stack_guard_write_fault
• test_mpu_thread_stack_guard_write_fault

[xuchang-vivo]

build_prs

[github-actions]

Job is started, see https://github.com/vivoblueos/kernel/actions/runs/22993852808.

[github-actions]

❌ Job failed. Failed jobs: check_format (failure), build_and_check_boards (failure), see https://github.com/vivoblueos/kernel/actions/runs/22993852808.

[xuchang-vivo]

build_prs

[github-actions]

Job is started, see https://github.com/vivoblueos/kernel/actions/runs/22994193994.

[github-actions]

✅ All jobs completed successfully, see https://github.com/vivoblueos/kernel/actions/runs/22994193994.